This is an attribute for the CMS concrete5.7 (http://www.concrete5.org/).
It will only allow you to define a text when configuring the attribute in the backend. The user will only see a static value you've defined.
This is probably only useful when you're building a dynamic application where the frontend form is generated by an automated process. This attribute makes it possible to add a hint, instructions or something similar to a dynamically created form.
Make sure you install the package in a directory called "attribute_plain_text" in your packages directory.